Optical memory has been very successful in certain areas, the most prominent being the Compact Disk (CD) involving playback (read only) of musical information. Although rewriteable optical media is being developed, for example phase-change and magneto-optical, other characteristics of optical recording technology have limited its use for computer related applications. One limitation is the rate at which data is read. This rate is limited by the spinning speed of the disk and the fact that only one source/detector i used. The other limitation is access time, or the average time it takes to access a randomly located bit of information. This time is limited by the mechanical motion of the head over large distances along the radius of the disk.
Increase of data rate has been the object of many efforts. One approach, to speed up the rotational rate of the disk, is limited in the fact that the disks already rotate at almost their maximum practical speed. Use of a shorter wavelength source will increase the data rate for a given rotational rate. A factor of 2 reduction in wavelength will increase the areal data density by 4 times; however the linear density, which is relevant to data rate, is only increased 2 times. To accomplish even this modest increase requires development of diode lasers emitting in the near ultraviolet, which will take considerable time.
Use of multiple read sources can increase the data rate by an order of magnitude or more, independently of the other two approaches. Multiple laser sources envisioned thus far have most often been linear, i.e., one-dimensional (1D), arrays of edge-emitting laser diodes. See, for example, the article by Carlin in Laser Focus World, July 1992, pp. 77-84, and by Marchant in "Optical Recording" (Addison-Wesley, Reading, Mass., 1990), both relating to optical disks, and by Bouldin and Drexler, U.S. Pat. No. 4,884,260, relating to optical tape. These systems have the disadvantage of astigmatic elliptical beams resulting from the use of edge-emitting laser diodes. Correcting such beams in an array is difficult. The edge-emitting laser diode geometry also does not allow the use of two-dimensional (2D) arrays, except by splitting the beams by, for example, diffraction gratings. MacAnally in U.S. Pat. No. 4,982,395 describes a composite optical grating which allows the simultaneous reading of 2 adjacent concentric tracks. Marchant also describes experiments conducted with a gas laser using a diffraction grating to produce 9 beams. But this was not a practical system because of the size and the difficulties of firming, aligning, modulating and maintaining of these beams. Moreover, the spacing between the focussed spots was too large to use with conventional CD media. The two referenced publications and the two referenced patents are herein incorporated by reference. None of the prior art known to us describes a practical 2D readout from optical recording media, nor does it describe a practical means for demagnification to make a 1D readout from with 4 or more beams.